This invention relates generally to a fault identification system useful in protective relays for an electric power system, and more specifically concerns such an identification system where electric current data measured and calculated by the protective relays at the ends of the power line are available to the other protective relays on the power line.
One well-known system for determining the existence of faults on electric power lines is line current differential protection. In line current differential protection, protective relays are positioned at the respective ends (terminals) of the protected power line. A relay at one end of the line (e.g. the local relay) will use current values which it measures at its end of the line in combination with current values which are measured by relays at the other end or ends of the power lines (the remote relays) to produce fault determinations.
Specifically, for a two-terminal line which is protected by a line differential system, the protection arrangement requires current measurements by protective relays at both ends of the line. The protective relays at the respective ends of the lines exchange magnitude and phase angle information of the currents they measure by means of a separate communications channel, such as a fiber-optic line (or other means). In a typical arrangement, each relay is capable of making a decision to trip a circuit breaker for the power line based on the current values it measures and the current values it receives from the relay at the other end of the line. Each relay has an associated breaker on the line which it (the relay) protects.
In making a fault-type determination on the protected line (an internal fault), it is important to identify which phase or phases of the three-phase power system are involved. Proper identification of fault type is important in making correct and appropriate tripping actions. For instance, in a protective relay which includes a single pole trip capability, when only a single phase is faulted, i.e. when there is a single phase-(line) to-ground fault, the proper action of the protective relay is to trip only the single pole in the circuit breaker associated with the particular faulted phase.
It is also important for the protective relay to provide information concerning the faulted phase. The information includes relay front panel LEDs (light emitting diodes) which identify the faulted phase, generation of event reports and fault-type identification output signals from the relay. Fault-type identification is important for other reasons. If the relay includes reclosing capability, it is important to be able to distinguish multiphase faults from single line-to-ground faults, because the success rate for reclosing multiphase faults is very low. Still further, when fault types are accurately identified, those computing algorithms in microprocessor-based relays which are not relevant for the identified fault can be blocked, saving computation time for the overall system.
Existing fault-type selection or determination systems have certain disadvantages. Existing systems often have difficulty differentiating between single line ground (SLG) faults, phase-to-phase (PH-PH) faults and phase-to-phase to ground (PH-PH-G) faults over a wide range of system conditions. Second, existing systems often have difficulty providing an accurate identification of a fault during complex fault conditions, such as for simultaneous ground faults, i.e. ground faults which occur simultaneously on different phases on adjoining power lines. Still further, existing systems are often severely influenced by system non-homogeneity or load flow, which affects their overall accuracy to making fault-type determinations.
Accordingly, the invention is a system for fault-type identification which is used in a protective relay for an electric power system. The system uses three phase system information from the power line, the system comprising: a first circuit means which is responsive to individual protective elements which recognize the presence of single line to ground (single phase) faults for phases A, B and C on the power transmission line, the first circuit means including a portion thereof for recognizing and asserting output indications of single line to ground faults, faults involving two phases, and three phase faults, upon the occurrence of different combinations of outputs from the protective elements; a calculation circuit responsive to an enable signal for determining the angular difference between the total zero sequence current and the total negative sequence current with a three phase current on the power line, for identifying high resistance faults, when the angular difference is in one of three pre-selected angular difference sectors, wherein an angle difference in the first sector indicates an A phase to ground fault or a BC phase-to-phase to ground fault, wherein an angle difference in the second sector indicates a B phase to ground fault or a CA phase-to-phase to ground fault, and wherein an angle difference in the third sector indicates a C phase to ground fault or an AB phase-to-phase to ground fault, wherein angle differences identified in each of the three sectors are indicated by sector signals; and processing means for determining which of the two possible indicated by the determined angular differences responsible for the fault and for providing a signal indication of said fault type.